Ink jet printers that utilize replaceable print heads, for example ink jet cartridges, often have problems precisely aligning a plurality of ink emitting nozzles, or jets, associated with each discrete ink jet cartridge which cannot easily be corrected through mechanical manipulation of a respective print head or a carriage to which the print head releaseably engages. These alignment problems become aggravated as the number of cartridges, and therefore the number of ink jet nozzles increase, as the spacing between the (furthest) jet nozzles increases, and resultant differing mounting locations among print heads following replacement of print heads. In prior art calibration techniques used to determine what adjustment is necessary typically require a printed test pattern, manual observation, entry of corrective values into print control circuitry, and retesting to confirm proper orientation among the several print heads.
In a wide format ink jet printer employing a subtractive color ink set, such as cyan, magenta, yellow, and black (CMYK) inks, typically a corresponding number of print heads are mechanically and electronically coupled to precisely locate, and therefore combine colored ink droplets to obtain a desired broad visual impression of color and tone, or color gamut, to a human viewer. The mechanical alignment among the print heads is referred to as "register" and "perfect registration" is a objective of the present invention herein, so that when desired colored ink droplets propelled from any of a plurality of ink emitting nozzles of each print head arrives at precisely the same location to thereby create a desired impression of a color to a human viewer. Because differences in primary ink colors are generally easily detectable by the viewer and provide ready demarcation points relative to the print head, it is common to provide a relatively simple calibration routine so that an end user may calibrate each of the print heads and their respective nozzles with respect to each other to obtain a defined acceptable range of dot placement. In addition, typically during manufacture of a wide format print engine a select number of quality assurance and other engineering verification tests help assure each print engine built meets or exceeds its specification so that an end user may later perform such relatively simple end user calibration routines and thus successfully calibrate and compensate for different positional location and alignment among several replaceable print heads. To efficiently and successfully implement such a manufacturing process requires an amount of flexibility in successfully adapting to unique build characteristics of each print engine. Thus a need exists in the art for adapting to build characteristics of cooperating subassemblies so that an end user may successfully utilize the aforesaid relatively simple calibration techniques without difficulty.
For example, the relatively simple calibration routine typically involves visual inspection of printed output from a print head that may have a cyan set of ink jet nozzles which are nominally a number of pixels horizontally offset from the black set of jets. A test pattern may be laid down by the cyan jets followed by test pattern laid down by the black jets. The test patterns may be compared to determine that, in actual operation, the black set of jets horizontally follows the cyan set of jets by a visually ascertainable number of pixels. In such a case, the timing of the firing of the black set of jets may be adjusted by the ascertained number of pixel locations, so that patterns laid down by the cyan and black jets will better horizontally match each other. Vertical calibration can be carried out in a similar way. Because calibration is an important part of properly aligned printing, the ink jet printing industry continually seeks new and better ways to readily determine what calibration adjustments are needed. In prior art bi-directional print engines having the ink jet nozzles of each print head aligned with the horizontal direction of carriage movement, the y-component can be ignored. In a drum based print engine, however, the y-component of motion contributed by the rotation of the drum member and thus that of a sheet of print media residing thereon must be considered. Furthermore, because the print heads in prior art bi-directional print engines travel over a common printing path relative to an elongate platen only a single narrow path upon the typically elongate platen must be compensated for surface vagaries and imperfections. In a drum-based color print engine, however, the entire surface of the drum can contribute error with simple surface imperfections as each of the print heads must traverse every point of an imageable area on the printing medium. Thus, a need exists for a relatively simple and efficient method of compensating for slight misalignment of cooperating subassemblies of drum based print engines and imperfections in the imageable surface of a print bearing surface portion of the drum member of such print engines.
Additional problems with prior ink jet head configurations involve the mounting of the print head for accurate placement and movement across the printed image. The rail structure for the print head must adequately support the print head not only over the entire printed image, but also for any cleaning, maintenance and other auxiliary functions of the print head. It is common for such a rail to experience what can be described as a sagging phenomena due primarily to the force of gravity, which adds positional error to a final desired printing location.
Furthermore, ink jet printers also need a consistent, accurate method to determine when the ink jets should be fired based on the location of each print head with respect to an image to be printed. Accurate positioning of ink dots on the printed image is necessary for accurate reproduction of the desired image. Prior art bi-directional printers have optically sensed markings from an encoder disk disposed along a linear path of carriage travel to determine print head position. The encoder disk markings are typically more or less evenly spaced apart from each adjacent marking other across the path of travel of the print head. Such an encoder disk reader produces an electrical signal as the print head changes location across the encoder disk, and the prior art ink jets are fired based directly on the timing of the encoder disk signal or are fired based on derivative of the timing signal. Further compensation for time of flight (TOF), and droplet velocity, etc. have been attempted to better describe the behavior of each ink droplet between the nozzle and the printing medium. Prior art encoder disks thus provide one way to determine when the ink jets should be fired in bi-directional prior art print engines. However, various errors prevent the encoder marking from corresponding exactly with the position of an ink dot on the image. These errors tend to be exacerbated as the speed of printing and size of output are increased. High-speed, large format printing requires a high degree of accuracy to generate quality graphics, and improved photorealistic output, so that a need exists for more accurate method of determining when to fire the ink emitting nozzles of a print head based on its location with respect to the image. In drum-based imaging systems, typically a rotary encoder coupled to an axis of rotation of a drum member provides a similar electrical signal which relates to an axial segment of the drum member. A phenomena known as "run out" relates to non-concentric alignment of center points of rotation only, and not to rotation induced error in themselves. This error is compounded where the circumference of the drum member is not perfectly aligned with an axis of rotation of a rotary motion encoder. In addition, the much greater circumference of the drum member compared to the rotary encoder, thus creating a mechanical disadvantage which inherently limits the absolute positional resolution of a control signal directed to the surface of the drum member and based on an output signal from the encoder, which defines a native positional accuracy of a print head control circuit using such an encoder.
Furthermore, in a drum-based print engine as taught herein where every subassembly must be field-replaceable to reduce cost and complexity of on-site diagnosis and repair, this rotary encoder might be separately replaceable from the drum member. Thus, positional error related to non-concentric alignment of the rotary encoder sensor and the axis of rotation of the drum member add complexity and positional error in addition to other sources of error which must be eliminated or compensated by suitable print head control circuitry and carriage drive software. Thus a means to increase the accuracy of such a prior art rotary position sensor in a multi-print head drum-based print engine exists in the prior art.
In the relevant drum-based imaging system prior art, a printing medium attaches to an exterior or interior surface of a rotating drum which rotates to pass the printing medium under one or more print heads, actuators, or sensors mounted on an carriage articulated in an axial direction parallel to the axis of rotation of the drum. The carriage then controllably traverses the width of the drum as the print media dispenses ink upon the print media. The combination of the spinning drum and traversing carriage assures that all print heads can access the full surface of the drum with a plurality of adjacent spiral-shaped print swaths. Thus a need exists in the prior art to correct these inherently skewed boundaries of printed output so that printed output will closely align with a sheet of printing medium appropriately applied to a drum member for receiving a printed image.
Also, prior art drums were typically manufactured to tightly controlled specifications and little if any compensation for dimensional variance in the imaging surface of the drum. Thus, inherent in the prior art is the expense involved in exploiting highly precise manufacturing processes that contribute cost and complexity to the fundamental component of drum-based imaging systems--the drum. Therefore, a need exists in the prior art to exploit low cost manufacturing materials and processes and compensate for surface deviations and inaccuracies and misalignment of subassemblies in relatively complex multi-print head wide format print engines. Furthermore, in this segment of the relevant prior art, which includes drum-based document scanner equipment, a known skewing effect inhibits the creation of truly square comers of the scanned image since absent modification prior art drum-based printing system imaging components inherently print continuous spiral swaths over the print media. In the prior art related to document scanners an image sensitive sensor receives the image in a similar manner, and thus encounter a similar difficulty. One prior art approach for this problem suggests use of a cam member coupled to a carriage drive mechanism so that at the end of a print sized to fill the surface of the drum the carriage is stepped sideways one swath thereby capturing the image with a series of adjacent, non-spiraling, swaths. Besides the obvious limitation inherent in such a design wherein only images sized to fill the printing area of the drum, this approach may not be dynamically modified to suit any particular image. Thus, a need exists in the art to solve this known limitation in the field of drum-based imaging devices, so that truly square comers of documents and images related thereto may be printed without further compensation.
Finally, while other known techniques for tracking and controlling the motion of the drum member, print heads, and carriage assembly might also include optical rotary encoders coupled to the axial shaft of a drum member, and in some circumstances on the carriage motor, the present invention addresses and overcomes the above-noted problems and difficulties in the prior art and offers a much higher level of compensation for variations in the drum surface, non-concentric alignment of a rotation sensor coupled to an axis of rotation of the drum, and changing cartridge placement thereby allowing more precise placement of multiple inks of differing color to form photorealistic full color digital images upon a single segment of media attached to the drum with ability to correct for a lower cost manufacturing process for the drum member and ultimately the entire drum based print imaging engine. The present invention further addresses the need to accurately propel ink from a plurality of nozzles resident on a multi-print head carriage assembly so that ink droplets accurately reach preselected locations on the print medium thereby generating photorealistic output from a relatively low cost computer controlled full color, wide format, ink jet digital drum-based print engine.